As one of the principles of a conventional acceleration sensor for detecting acceleration in the substrate thickness direction, there is a method for detecting a change in electrostatic capacitance in accordance with the acceleration. As an acceleration sensor based on this method, an acceleration sensor (an acceleration sensing motion converter) including, for example, a torsion beam (a deflecting part), an inertia mass body (a weight), a detection frame (an element), and a detection electrode (a sensing electrode) as main components has been known (see for example Japanese Patent Laying-Open No. 05-133976 (Patent Document 1)).
The acceleration sensor of Patent Document 1 has one detection frame having a surface facing a substrate. The inertia mass body is provided on one end part of the detection frame. The detection frame is supported on the substrate so as to be rotatable with the torsion beam as the axis of rotation. The detection electrode for detecting this rotational displacement is provided under the detection frame.
Further, an acceleration sensor having an inertia mass body disposed in the same plane as a detection frame, instead of being disposed on a detection frame, has been known (see for example International Publication WO 2003/044539 (Patent Document 2)). The acceleration sensor of Patent Document 2 includes a torsion beam, an inertia mass body (a mass body), a detection frame (a movable electrode), and detection electrodes (first and second fixed electrodes). The torsion beam is coupled to an anchor part supported by a substrate. One detection frame (movable electrode) is coupled to the torsion beam, and supported on the substrate so as to be rotatable with the torsion beam as the axis of rotation. Link beams are provided on one end part and the other end part of the detection frame, at positions a predetermined distance away from the center line of the detection frame. The inertia mass body (mass body) is coupled to the link beams. The inertia mass body is constituted to be movable in accordance with acceleration in the thickness direction of the silicon substrate.
When acceleration in the substrate thickness direction is applied to the acceleration sensor constituted as described above, inertia force in the substrate thickness direction acts on the inertia mass body. Since the inertia mass body is provided on the one end part, that is, at a position deviated from the axis of rotation in the substrate in-plane direction, this inertia force acts on the detection frame as a torque around the torsion beam. As a result, the detection frame is rotationally displaced. The distance between the detection frame and the detection electrode is changed by this rotational displacement, which causes electrostatic capacitance formed by the detection frame and the detection electrode to be changed. The acceleration is measured from this change in electrostatic capacitance.    Patent Document 1: Japanese Patent Laying-Open No. 05-133976    Patent Document 2: International Publication WO 2003/044539